A cyclotron is a type of circular particle accelerator in which negatively or positively charged particles are accelerated outwards from the centre of the cyclotron along a spiral path up to energies of several MeV. Unless otherwise indicated, the term “cyclotron” is used in the following to refer to isochronous cyclotrons. Cyclotrons are used in various fields, for example in nuclear physics, in medical treatment such as proton-therapy, or in radio-pharmacy. In particular, cyclotrons can be used for producing short-lived positron-emitting isotopes suitable for PET imaging (positron emitting tomography) or for producing gamma-emitting isotopes, for example, Tc99m, for SPECT imaging (single photon emission computed tomography).
A cyclotron generally comprises several elements including an injection system, a radiofrequency (RF) accelerating system for accelerating the charged particles, a magnetic system for guiding the accelerated particles along a precise path, an extraction system for collecting the thus accelerated particles, and a vacuum system for creating and maintaining a vacuum in the cyclotron.
A particle beam constituted of charged ions is introduced into a gap at or near the center of the cyclotron by the injection system with a relatively low initial velocity. As illustrated in FIG. 3, this particle beam is sequentially and repetitively accelerated by the RF accelerating system and guided outwards along a spiral path comprised within the gap by the magnetic field generated by the magnetic system. When the particle beam reaches its target energy, it can be extracted from the cyclotron by the extraction system provided at a point of extraction, PE. This extraction system can comprise, for example, a stripper consisting of a thin sheet of graphite. For example, H− ions passing through the stripper lose two electrons and become positive. Consequently, the curvature of their path in the magnetic field changes its sign, and the particle beam is thus led out of the cyclotron towards a target. Other extracting systems exist which are well known to the persons skilled in the art.
The magnetic system generates a magnetic field that guides and focuses the beam of charged particles along the spiral path until it is accelerated to its target energy. In the following, the terms “particles”, “charged particles”, and “ions” are used indifferently as synonyms. The magnetic field is generated in the gap defined between two magnet poles by two solenoid coils, 14, wound around these poles. Magnet poles of cyclotrons are often divided into alternating hill sectors and valley sectors distributed around a central axis. The gap between two magnet poles is smaller at the hill sectors and the larger at the valley sectors. A strong magnetic field is thus created in the hill gap portions within the hill sectors and a weaker magnetic field is created in the valley gap portions within the valley sectors. Such azimuthal magnetic field variations provide radial and vertical focusing of the particle beam every time the particle beam reaches a hill gap portion. For this reason, such cyclotrons are sometimes referred to as sector-focusing cyclotrons. In some embodiments, a hill sector has a geometry of a circular sector similar to a slice of cake with a first and second lateral surfaces extending substantially radially towards the central axis, a generally curved peripheral surface, a central surface adjacent to the central axis, and an upper surface defining one side of a hill gap portion. The upper surface is delimited by a first and second lateral edges, a peripheral edge, and a central edge.
It is common to have more than one point of extraction for extracting the particle beam from more than one point of a same hill sector of a cyclotron. An advantage of having at least a second extracting point within a same hill sector is to provide a fall-back extracting point in case the first extracting point is out of order and thus ensuring continuity of service. Another advantage is to lead the same ions with same energy towards two different targets at the same time, by positioning a first stripper so that it intercepts only a fraction of the particle beam, and the second stripper so that it intercepts part or all of the remaining fraction of the particle beam which passed next to the first stripper. One major issue, however, with at least two points of extraction is to ensure that the particle beam being extracted from a first point of extraction has the same properties (energy, focusing, etc.) as the one being extracted from a second point of extraction. To date, this issue has not been resolved satisfactorily yet.
There therefore remains a need in the art to provide an isochronous sector-focused cyclotron wherein the optical properties of an extracted particle beam do not depend on the position of a point of extraction located in a hill gap portion. This is particularly advantageous for cyclotrons having at least first and second points of extraction at a same hill sector, wherein the particle beam extracted from the first point of extraction has the same properties as the one extracted from the second point of extraction.